My house was supposedly built circa 1945 (it's concrete block on a slab) and I'm guessing that the furnace/ac registers in the walls are of an earlier style that may be outdated. (I've attached a couple of pics) They have sets of upper and lower louvers, the ducting coming down from the attic inside the wall, and the tube terminates just below the upper louvers. The entire inside of this large register assembly is open which looks like it allows airflow to exit either upper or lower louvers.

I'm wondering if there's any reason that these register assemblies couldn't or shouldn't be replaced by the typical 6x12 register or something similar. Since we're doing some remodeling anyway, I would prefer to get rid of these large, garish things and put something less intrusive and a bit more current in the lower area of the wall.

Since I'm far from an expert on these things I want to make sure I'm not overlooking something. Any thoughts?

There are a total of five of these throughout the house in various rooms. Since the house had a high-efficiency furnace and central air when I bought it in 2000 (and still has the same system), I'm certain that it had been converted from some other kind of earlier heating system at some point in time, but that they must have retained these registers. My main concern at this time is just whether I can replace these things with the smaller and more current type of register without causing unforeseen issues.

I would be looking to get ri8d of them too. The question will be how they were installed, tyhey may have a flange behind the brick. Which you would remove with a sawsall with a blade for cutting metal.